We're going to learn more now about Kirstjen Nielsen. She's the U.S. secretary of Homeland Security and the face of the Trump administration's policy of separating families at the U.S. border. She went before reporters last night at the White House, and here's what she said when asked whether she wants the image of the U.S. to be, quote, "children in cages."

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KIRSTJEN NIELSEN: The image that I want of this country is an immigration system that secures our borders and upholds our humanitarian ideals. Congress needs to fix it.

CORNISH: NPR national security correspondent Greg Myre has been looking into Nielsen's background, and he finds this isn't the first time she's been deeply involved in a major government controversy. Greg is in the studio now. Hey there, Greg.

GREG MYRE, BYLINE: Hey there, Audie.

CORNISH: So Kirstjen Nielsen worked at the White House under President George W. Bush. What was her role back then?

MYRE: So her title was director of response and planning for the Homeland Security Council in the Bush White House. This means she was right in the thick of things as the administration responded to Hurricane Katrina in New Orleans in 2005. And, as we know, the Bush administration was savaged for what was seen as a very slow and inadequate response. There was a Senate report afterword - ran hundreds of pages. It cited Nielsen by name. It said she had received an email six months before warning about the concerns about flooding in New Orleans. And then even in the days before it struck and then in the aftermath, the response was really inadequate. So that's part of her background.

CORNISH: Before that, what was she doing? What have you learned?

MYRE: Well, she's from Florida originally, and then has really made her career in Washington - went to Georgetown University, University of Virginia Law School. After she left the Bush administration, she stayed in Washington, created a cybersecurity firm, took a fellowship at George Washington University. She came back into government with the Trump administration very much attached to John Kelly. She was his chief of staff when he started out at Homeland Security last year - followed him to the White House when he went there. Then went back to DHS and has been there as the secretary since January.

CORNISH: So she's very much a known quantity in Washington. What's her reputation?

MYRE: Smart, ambitious, hard-charging. There have been concerns that she hasn't run a large organization. DHS is such a sprawling entity with a quarter-million employees. It includes everything from Customs to the Secret Service, TSA, the Coast Guard. And it's been run by people like John Kelly, a four-star general, or former governors. So there was this concern as she took over at the department.

CORNISH: In terms of her own agenda, what did she bring to the department?

MYRE: Well, she is very much - whether it's her agenda or not - focused on immigration. Now, previous Homeland Security chiefs have maybe focused on terrorism but - under this administration - very much about immigration. And we heard this yesterday in this very contentious press conference.

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NIELSEN: Here is the bottom line. DHS is no longer ignoring the law. We're enforcing the laws as they exist on the books. As long as illegal entry remains a criminal offense, DHS will not look the other way.

CORNISH: Briefly, what do we know about Trump's opinion of her?

MYRE: Well, it was reported that he berated her at a Cabinet meeting, and she almost quit last month. And, you know, she's a person who's been in the background - the staffer who made the trains run on time. But in this job in this administration, she's going to be the face of immigration and is going to face a lot of pushback.